Effect of Water Microsolvation on the Excited-State Proton Transfer of 3-Hydroxyflavone Enclosed in γ-Cyclodextrin.

Khanittha Kerdpol,Supawadee Namuangruk,Peter Wolschann,Rathawat Daengngern,Nawee Kungwan,Thanyada Rungrotmongkol,Chanchai Sattayanon,Supot Hannongbua

doi:10.3390/molecules26040843

Abstract

The effect of microsolvation on excited-state proton transfer (ESPT) reaction of 3-hydroxyflavone (3HF) and its inclusion complex with γ-cyclodextrin (γ-CD) was studied using computational approaches. From molecular dynamics simulations, two possible inclusion complexes formed by the chromone ring (C-ring, Form I) and the phenyl ring (P-ring, Form II) of 3HF insertion to γ-CD were observed. Form II is likely more stable because of lower fluctuation of 3HF inside the hydrophobic cavity and lower water accessibility to the encapsulated 3HF. Next, the conformation analysis of these models in the ground (S0) and the first excited (S1) states was carried out by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, respectively, to reveal the photophysical properties of 3HF influenced by the γ-CD. The results show that the intermolecular hydrogen bonding (interHB) between 3HF and γ-CD, and intramolecular hydrogen bonding (intraHB) within 3HF are strengthened in the S1 state confirmed by the shorter interHB and intraHB distances and the red-shift of O–H vibrational modes involving in the ESPT process. The simulated absorption and emission spectra are in good agreement with the experimental data. Significantly, in the S1 state, the keto form of 3HF is stabilized by γ-CD, explaining the increased quantum yield of keto emission of 3HF when complexing with γ-CD in the experiment. In the other word, ESPT of 3HF is more favorable in the γ-CD hydrophobic cavity than in aqueous solution.

Highlights

Fluorescent organic molecules with strong intramolecular hydrogen bonds connected by proton donating and accepting groups have gained considerable attention in recent years owing to their unique fluorescent emission in term of large Stokes shift without self-absorption [1]
It can be observed that the intramolecular hydrogen bonding (intraHB) and intermolecular hydrogen bonding (interHB) are strengthened in the S1 state confirmed by the shorter distances of important bonds involving in the excited-state proton transfer (ESPT) process, the red-shift observed by IR vibrational spectral calculations, and a high value of EHB from topology analysis of the electron density
The strength of all intraHB and interHBs was further confirmed by the red-shift values of the O–H stretching vibrational frequencies between the S0 and S1 states from the simulated IR spectra, together with the topology analysis at bond critical point (BCP) from quantum theory of atoms in molecules (QTAIM) performed by Multiwfn [68], which was employed in previous studies [69,70,71]

Summary

Introduction

Fluorescent organic molecules with strong intramolecular hydrogen bonds (intraHBs) connected by proton donating and accepting groups have gained considerable attention in recent years owing to their unique fluorescent emission in term of large Stokes shift without self-absorption [1]. Their unique properties harnessing from the excited state intramolecular proton transfer (ESIntraPT) could be typically described by the characteristic four-level photocycle.

Structural Optimizations

O–H Stretching and Topology Analysis

Frontier MOs Analysis and Simulated Spectra

Molecular Dynamics Simulations

Quantum Chemical Calculations

Conclusions